The present invention relates to a data compression method for reducing the amount of large-volume image data such as a medical X-ray image data, or more in particular to an image data compression method which is capable of storing information of high utility better than information of low utility and in which the information of high utility is compressed with higher ratio than that of low utility.
Conventionally, the information of medical X-ray image generated in medical institutions or the like, for example, have been used and stored as an X-ray film. In recent years, however, in order to reduce the storage space or facilitate picture search thereof, or to process an image by use of an image processing technique, an image processing system has been under development for reading an X-ray film image optically, converting it into a digital electrical signal by photoelectric conversion means and analog-digital conversion means, and storing it in a memory unit like an optical disk or subjecting it to an image processing computation by electronic computer.
Such an image processing system according to the related art, as described in the specification of JP-A-61-231 and shown in FIG. 4, comprises a film image reader 1 for converting an X-ray film image data into a digital signal and an image processing unit body 2. In the drawing, an output digital signal of the film image reader 1 reaches an image processing section 4 through a measurement interface 3 in the image processing unit body 2. The output digital signal is subjected to a measurement process for converting the information on light intensity to a density data at the image processing section 4, and is stored in a storage unit 7 including a magnetic disk unit, an optical disk unit, a large-capacity semi-conductor memory unit or the like. The image data thus stored is transferred to a display image memory 6 as required, and displayed as an image on an image display unit 9 directly connected to the memory 6. In the process, the image data may alternatively be transferred to the image processing section 4, and after being subjected to various image processing computations, may be transferred to the display image memory 6. The image memory 5 is for storing the image data temporarily at the time of the measurement processing or the image processing computation of the data by the image processing section 4. The whole of this image processing unit is operated by an I/O unit 10 connected to the image processing unit body 2 via an I/O unit interface 8.
The capacity of the image data handled in this image processing unit reaches as much as approximately 10M byte for each sheet of X-ray film. An enormous storage capacity would thus be required if the data are to be stored in their direct form. In this image processing unit, therefore, the data is compressed and stored in the storage unit 7 by use of various data compression methods including a coding method by dividing an image into uniformly sized blocks (hereinafter referred to as "the non-uniform block coding" as discussed in "The Preliminary Texts for the 1987 National Conference in Commemoration of the 70's Anniversary of the Institute of Electronics, Communications and Information Engineers of Japan" (published 1987, at pp. 5 to 50). The conventional image processing unit shown in FIG. 4 is so configured that the image data compression is effected by an image processing section 4 using an image memory 5, and the processing time is shortend by a configuration in which the image data compression process may be effected in parallel to the processes for data measurement and compilation and transfer of the image to the display image memory 6.
In the image data compression effected in this conventional image processing unit, an irreversible data compression method such as a non-uniform block coding is often used in order to increase the compression ratio. If an attempt is made to obtain a high compression ratio, therefore, the fidelity of the reproduced image information reconstructed from the compressed data as compared to the original image information is necessarily reduced.
The irreversibility means that when a compressed data is reproduced, a reproduced image cannot be completely reproduced and has error in comparison with original image.
The non-uniform block coding is a method in which portions of the image with a small optical density (brightness) change unified in a large block and portions of the image with a large optical density change unified in a small block. The interior of each block is approximated by a bilinear function, with the form of block division described by a quad-tree.
With regard to the image data handled by the image processing unit, the values of the image data providing important information are not distributed uniformly over the range of available values in many cases but limited to a region of specific values therein.
In the case of a medical X-ray film image handled by the conventional image processing unit shown in FIG. 4, in which a local density of the film is read out as an image data, for example, the important information is often included in the values in the region from low to middle densities. This is due to the fact that a film is exposed in such a manner as to contain important medical information in the region from low to middle density high in visual ability of the observer to discriminate the density. The low-density region, in particular, contains an image of internal organs superimposed one on another, and a minor density deference in the low-density region often provides important information indicating the edges of superimposed organs. The high-density, on the other hand, often contains useless information such as the direct X-ray and its scattered beam or a noise of the film image reader 1.
As mentioned above, the conventional irreversible data compression method poses the problem that since the important and useless information are compressed without being discriminated but with a substantially uniform fidelity, the important information may be dropped while useless information may be stored.